Operations such as road making and terrain forming require the use of special equipment and the ability to precisely monitor and control the location and orientation of such equipment. Typical of such special equipment is a machine which provides a motive power unit upon which is mounted a pivoted working tool such as a scraper blade, rake, or bucket. The most commonly used special equipment of this type is a grader, which comprises a body which includes a motive power unit, and a pivoted working tool. The present invention will be described with special reference to a grader, but it should be appreciated that the method of the invention is in no way limited to a grader, but is applicable also to any machine of the above described type, such as bulldozers.
When ground is being worked with machines of this type, it is the location and orientation of the motive power unit and the working tool which determine which part of the terrain will be formed. For example, in the case of a grader, the orientation and trajectory of the grader blade will determine where from, and in which direction, earth is moved. In the past the orientation of the working tool and the orientation of the axis of the machine have been determined by eye by the driver, based on experience. However, this means that the quality of the finished work is very dependant on the skill of the driver, and in an effort to achieve more predictable results, there has been a recent move to provide automated assistance to the driver. In order to monitor the location and orientation of the motive power unit and the working tool at all times, a number of sensors may be used. For example, a 3D sensor such as a Global Positioning System (GPS) or robotic total station (RTS) target may be positioned at each end of the working tool. From the combination of this data, the heading of the working tool and the orientation of the motive power unit and of the working tool can be determined by various methods.
Alternatively, the combination of a rotational sensor, placed where the working tool connects to the motive power unit in order to measure the angle between the two, and a single 3D sensor on the working tool may be used, but this gives a significantly less accurate result.
In practice, RTS is preferred to GPS because it gives more accurate results on the scale of use. RTS uses a target on the working tool, which has one or more prisms to reflect light back to the instrument for measurement. As the RTS target moves, servos turn the instrument to automatically keep track of the target. RTS measures both angles in the horizontal plane and the elevation of the target. It has an electronic distance meter which can precisely measure the distance from the instrument to the target using laser technology.
The use of multiple 3D sensors increases the cost of the equipment, and can also give rise to problems such as incorrect target recognition or interference between the 3D sensors. Therefore it is desirable for improved accuracy (as well as for economy) to reduce the number of 3D sensors needed.
A 3D sensor at only one end of the working tool will provide sufficient information to calculate the orientation of the machine when the machine is travelling in a straight line. In this case, the machine orientation is the same as the machine heading, and is parallel to the heading of the working tool. However, the known model, which utilises only a straight-line fit, breaks down when the machine trajectory changes from a straight line to a curve. In practice, the machine trajectory seldom is restricted to a straight line:—typically, a machine such as a grader moves in a complex trajectory which incorporates many curves. For this type of work, the known model gives very poor accuracy.